The purpose of this project is to determine whether the insulin- like peptide hormone, somatomedin, is a stimulant of fetal growth. To date, a sensitive, specific competitive membrane binding receptor assay for somatomedin has been developed, and the interaction of somatomedin with the hormone receptor of human placenta has been characterized. Somatomedin levels in cord blood from human fetuses at delivery have been quantitated. Somatomedin levels have been found to be in the low normal range of the child and the adult human. Samples of fetal pig plasma have been collected throughout gestation, and somatomedin levels are to be determined at an early date. Membranes are being prepared from fetal pig tissues in order to define the ontogeny of somatomedin receptors in the fetus. Studies of the rate of disappearance of I125- somatomedin from the plasma of rats have been completed in preparation for a study of the transfer of somatomedin from mother to fetus and fetus to mother.